The embodiments described herein relate generally to a nuclear quadrupole resonance (NQR) detection system and, more particularly, to an NQR detection system for reducing environmental radiofrequency interference signals in a detection signal generated by the NQR system.
NQR is a radiofrequency (RF) spectroscopic technique that may be used to detect a presence of materials containing quadrupolar nuclei, such as nitrogen-14, potassium-39, chlorine-35, and chlorine-37, that may indicate a material of interest is present. As used herein, the term “material of interest” refers to explosives, narcotics, home-made explosives (HME), and/or any other material that may compose a threat in an inspected region. NQR has been used for baggage and parcel screening, narcotics detection and/or explosives detection, such as detection of buried Improvised Explosives Devices (IED), personnel screening, and/or landmine detection.
At least some known NQR systems include an RF transmission device that transmits waves in the RF portion of the energy spectrum at the NQR frequencies associated with the materials of interest. NQR arises from the electrical interaction between the electric quadrupole moment of the NQR-active nuclei and the electric field gradient at the position of these nuclei created by the electrical charge distributions in the molecules of the material of interest. The transmitted RF waves excite transitions between energy levels defined by the electrical interactions. When the nuclei transition back to the equilibrium state, an NQR response is received from the nuclei. Such known NQR systems also include a receiving device that receives the NQR responses with the resonant frequencies. A material to be scanned is positioned in or near a tuned, resonant inductive element (usually referred to as a “coil”) that detects NQR signals induced by pulsed RF excitation fields.
In some applications of NQR, a sensor, such as an NQR coil, operates unshielded or partially shielded from electromagnetic (EM) fields. However, such a sensor may suffer from low signal-to-noise ratios (SNR), which may be further aggravated by a presence of external or background radio frequency interferences (RFI). The RFI may be caused by far away sources (i.e., radio stations) and/or from the presence of other equipment in the vicinity of the sensor (i.e., electronic and electrical equipment). In order to operate with low false alarm rate (FAR) levels when the sensor is deployed outside shielded enclosures, it is desirable that the NQR sensor be insensitive or immune to the presence of external RFI and/or environmental RF noise.
At least one known sensor design for improving rejection of environmental interferences includes gradiometer coils. The gradiometer coils are immune to EM fields that are uniform in space. As such, the gradiometer coils are sensitive only to a spatial derivative of the EM fields. In addition, such environmental interference may also include significant gradients that have magnitudes large enough to not be fully canceled by the gradiometer coils.
Another known sensor is a gradiometer that includes two separate coils wound in opposite directions and connected in series. Alternatively, the two coils are wound in the same direction but a phase inversion is performed in one of the coils before the signals are combined at a receiver. Noise that is detected by the two coils arrives at the receiver as two signals with opposite phases, leading to self-cancellation of the noise. A sample is always placed closer to one coil than to the other coil such that a NQR signal of the sample is not cancelled. However, this sensor has the disadvantage of reducing the SNR because the second coil adds thermal noise to the NQR signal upon summation of the signals.
Further, known research has proposed the use of excitation RF pulse sequences with composite pulses for cancellation of spurious signals. However, the use of such excitation RF pulse sequences results in significant signal-to-noise degradation that adversely impacts the detection performance of an NQR sensor implementing the excitation RF pulse sequences.
At least one known portable NQR system (i.e. an NQR wand, a backpack mine detector, and/or a landmine detector) uses a set of ancillary antennas or coils, such as a three antennas, for active RFI cancellation. The ancillary antennas are independent of a transmitting/receiving NQR sensor, such as being positioned several feet away from the receiving NQR sensor. The ancillary antennas sample three perpendicular components of external EM radiation that may interfere with the operation of the receiving NQR sensor. The ancillary antennas may be referred to as “RFI antennas” and are separated from the receiving NQR sensor (the “main NQR sensor”) and are located at a sufficient distance from the main NQR coil to avoid interferences between the RFI antennas and the main NQR coil. Such an NQR system provides relatively good performance in RFI cancellation but does not achieve the RFI rejection desired when the interferences do not correlate, for example, when the source of RFI is closer to the main NQR coil and/or when there are multiple paths/sources of RFI.
Phased-coil arrays are known for use in Magnetic Resonance Imaging (MRI) to improve spatial resolution and/or SNRs. In phased-coil arrays, nuclear magnetic resonance (NMR) responses from different surface coils within the array are combined to produce a single composite NMR image of the total sample. In at least one known phased-coil array, problematic interactions among nearby surface coils of the array are substantially reduced by overlapping adjacent coils to provide zero mutual inductance between adjacent coils and by attaching low-input-impedance pre-amplifiers to each of the coils, thus eliminating interference among next nearest and more distant neighbors. A phased array of coils allows simultaneous acquisition of multiple signals with minimal interference between them. However, each coil of the phased array receives NMR responses from a scanned object and any RFI near the scanned object because each coil of the array transmits and receives signals.